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DNV GL Headquarters, Veritasveien 1, P.O.Box 300, 1322 Høvik, Norway. Tel: +47 67 57 99 00. www.DNV GL.com
[Legal information] OTG-08 UWILD guidance May 2014 rev 1.0.docx
GUIDELINE ON UWILDOFFSHORE TECHNICAL GUIDANCE OTG-08, MARCH 2014
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TABLE OF CONTENT
Guideline on UWILD ..................................................................................................................... 1
Offshore Technical Guidance OTG-08, March 2014 ........................................................................... 1
Table of Content .......................................................................................................................... 2
1 Introduction ......................................................................................................................... 4
1.1 Introduction ................................................................................................................. 4
1.2 Objective ..................................................................................................................... 4
1.3 Scope .......................................................................................................................... 4
1.4
Structure ..................................................................................................................... 4
1.5 References ................................................................................................................... 4
1.6 Abbreviations ............................................................................................................... 5
2 Background ......................................................................................................................... 6
2.1 Regulatory Basis ........................................................................................................... 6
2.2 Position DNV GL ............................................................................................................ 6
2.3 Risk assessment ........................................................................................................... 6
3
Approach ............................................................................................................................. 8
3.1 General........................................................................................................................ 8
3.2 In-service Inspection plan (IIP)....................................................................................... 8
3.3 Flag Authorities............................................................................................................. 8
3.4 Survey Reporting .......................................................................................................... 8
3.5 Conditions .................................................................................................................... 9
4 Inspection scope ................................................................................................................ 11
4.1
General...................................................................................................................... 11
4.2 Applicable for all MOU .................................................................................................. 11
4.3 Column-stabilized units ................................................................................................ 12
4.4 Ship-shaped units ....................................................................................................... 13
4.5 Self-elevating units/Jack-ups ........................................................................................ 14
4.5.1 Introduction ........................................................................................................... 14
4.5.2 Separate surveys .................................................................................................... 14
4.5.3
Hull/Deckbox .......................................................................................................... 15
4.5.4 Leg and Spudcan ..................................................................................................... 15
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4.5.5 Long-term operation at location ................................................................................ 16
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1 INTRODUCTION
1.1 Introduction
Statutory and class regulations requires two bottom surveys in a 5 year period to inspect integrity and
water tightness of the hull. So called ‘Under water inspections in lieu of drydock’ (UWILD) survey may
replace the traditional drydocking related to these bottom surveys.
1.2 Objective
The objective of this guideline is to clarify to owners and Flag State Administrations the conditions for
which DNV GL to accept and support in-water bottom survey of Mobile Offshore Units (MOUs) as analternative to drydocking.
1.3 Scope
This guideline summarizes the scope expected to be covered by the UWILD for different types of MOUs,
including column stabilized, self-elevating and ship-shaped units.
The guideline is for the units covered by DNV GL Offshore Classification, including but not limited to
drilling units, FPSO’s, FSO and LNG production and storage units.
The guideline is line with the relevant unified requirements of the International Association of Class
Societies (IACS).
1.4 Structure
Section 2 describes the background of the UWILD approach; the regulatory basis, the position of DNV GL
and a risk assessment for acceptance of the approach for the described units.
Section 3 outlines the approach of DNV GLs UWILD based on four specific elements/conditions.
Section 4 describes the UWILD inspection scope, both generic as applicable for all hull types and as
specific for successively ship-shaped, column stabilized and self-elevating units. For the last are specific
considerations discussed, taking into account their specific fixed/floating nature and operation profile.
The document is completed with example of a UWILD planning document in Appendix A.
1.5 References
This guidance is based on the international regulations, standards and DNV-GL Offshore Service
Specifications/ Rules as listed in Table 1.
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Table 1 References
Reference1 Title
IMO MODU Code IMO Code for the Construction and Equipment of Mobile Offshore
Drilling Units
IMO SOLAS IMO International Convention for the Safety of Life at Sea
IACS UR Z15 International Association of Classification Societies’ Unified Requirement
Z15, Survey of Mobile Offshore Drilling Units
DNVGL-OSS-101 Rules for Classification of Offshore Drilling and Support Units
DNVGL-OSS-102 Rules for Classification of Floating Production, Storage and Loading
Units
DNVGL-OSS-103 Rules for Classification of Floating LNG/LPG Production, Storage and
Loading Units
DNVGL-OSS-104 Rules for Classification of Self-Elevating Units
1.6 Abbreviations
The following abbreviations are used in the document.
Table 2 Abbreviations
Abbreviation Description
CSU Colum Stabilized Unit
IACS International Association of Class Societies
IIP In-service Inspection Plan
MOU Mobile Offshore Unit
ROV Remotely Operated Vehicle
SEU Self-elevating unit
SSU Ship-shaped unit
UWILD Under Water inspection In Lieu of Drydocking
1 It is mentioned that the referred DNV GL service documents are subject to re-numbering as part of the DNV GLintegration
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2 BACKGROUND
2.1
Regulatory Basis
Bottom surveys are governed by the following regulatory basis:
According to IACS UR Z15, 4.1.1 and IMO MODU Code 1989 and 2009, 1.6.1.5, MOUs shall be
subjected two drydock surveys during any five-year period whereof the intervals between any two
such surveys should not exceed 36 months;
Further according to IACS UR Z15, 4.2 and Appendix A and IMO MODU Code 1989 and 2009,
1.6.2.5, an Administration may allow Underwater Inspections in Lieu of a Drydock survey (UWILD),
often called in-water bottom survey or bottom-survey afloat, provided that they are satisfied that
such an inspection is equivalent to a drydock survey;
The IMO SOLAS Ch. I, Reg. 10(a)(v) requires two bottom surveys in a 5 year period with the
interval period not exceeding 36 months.
The DNV GL Offshore Service Specifications governing the classification of MOUs require two
bottom surveys in the course of 5 years, and one of these is to be done together with the Renewal
Survey.
It is underlined that this regulatory basis implies acceptance by the Flag State Administration for any
UWILD approach as an alternative to drydocking for the required bottom surveys.
2.2 Position DNV GL
As a Recognized Organization (RO), DNV GL acts on behalf of the Flag State Administration in carrying
out inspections and surveys related to the statutory certification as, among others, governed by SOLAS
and MODU code as referenced above. DNV GL have long term experience with such arrangements and
consider an equivalent solution to be fully acceptable for MOUs provided that specific criteria with regard
to the condition of the unit and the alternative inspection arrangements are met, ref. Section 3 and 4 of
this guideline.
2.3
Risk assessment
The main reason to consider UWILD for Mobile Offshore Units compared to trading ships is their
exposure to a lower risk profile related to damages to the hull;
Main operations are in open sea and fixed locations, minimizing risk of grounding and fouling of
propellers and thrusters
By nature of design corrosion protection systems including anodes and / or impressed current
systems are installed to cover periods exceeding 5 years / design life. The system performance of
which may be monitored by measurement equipment / ROV surveys.
Design life of Production units stationed permanently at location in most cases exceeds the five
year certification period.
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MOUs normally operate in open waters with good visibility, making bottom survey by ROV/divers
well suitable
In addition to the lower specific risk profile as listed above, DNV GL Classification implies a tight,
controlled and transparent follow up on the conditions of the unit under class. In accordance with DNV
GL procedures, In-service Inspection Plans (IIPs) are established for all MOUs which specify the complete
inspection scope according to class rules. Subsequently, the control of the structural integrity of the
MOUs is ensured through regular class surveys according to the IIP’s.
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3 APPROACH
3.1 General
The approach of DNV GL to UWILD is based on the following elements:
1. Inspection of the MOU following the In-Service Inspection Plan
2. Acceptance of flag authorities for the UWILD approach
3. Survey report clearly stating the accreditation criteria of the bottom survey
4. The fulfillment of a set of specific conditions
The elements are further described in the following sub-sections. The requirements for the actual
execution of the UWILD scope are given in Section 4.
3.2 In-service Inspection plan (IIP)
The scope of work for inspection of MOUs is covered by the In-service Inspection Program (IIP) which
will contain the structural items to be surveyed to satisfy the requirements of main class including
watertight and structural integrity. Requirements to this can be found in DNVGL-OSS-101, OSS-102,
OSS–103 or OSS-104. The IIP constitutes the formal basis for surveying structural items under main
class and shall be completed to the satisfaction of attending surveyor before periodical survey can be
credited.
3.3 Flag Authorities
DNV GL Offshore Classification will address relevant flag authorities for their comment and general
acceptance to the practice outlined in this guideline.
As stated in Table 3, a request shall be made to the Administration when UWILD survey is intended to be
carried on an MOU as an alternative to inspection in drydock.
3.4 Survey Reporting
In connection with the survey reporting, the criteria on which the bottom survey has been credited shall
be clearly stated, referencing the acceptance from flag authorities (as applicable) and statement from
owner/master as specified in Table 3.
For non-completed scope for spudcan/mat of jack-ups, please refer to Section 4.5.4.
Note:
MODU Code 1979 : The bottom survey is not required for units
covered by the 1979 IMO MODU Code.
Norway – NMA: NMA do not specify any requirement for
intermediate bottom survey.
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3.5 Conditions
The conditions to be fulfilled for DNV GL to grant acceptance for UWILD are listed in Table 3.
Table 3 General survey scope for bottom survey of MOUs
Condition Description
Planning of in-
water survey
The owner shall submit plans and procedures for survey of the under-water hull to DNV GL
in advance of the survey and make these available on board.
Plans should include drawings or forms for identifying the areas to be surveyed, the extent
of underwater cleaning, NDT locations and methods, and provision for reporting of any
damages or deteriorations found.
See Appendix A of this document for example of survey plan table of content.
Plans should be in place for how to handle the discovery of cracks and damages which may
require repair at yard or in dock.
The owner should consider the implications of the choice of inspection method with regard
to the inherent limitations and possible need for supplementary inspection
In case of consecutive UWILD surveys the history of prior surveyed areas shall be available
in the plan.
Request to
Administration
DNV GL shall forward an application from the MOU owner/operator to the Administration
(flag state), seeking exemption from the drydocking requirements.
The application shall contain the owner’s survey plans and procedures reviewed and
accepted by DNV GL that the alternative arrangement will provide equivalent results to
drydocking.
It is assumed that the criteria set forth in the plans are covered by the owner when
preparing for the in-water survey.
Diving / ROV
company
The diving company shall be certified as a Service Supplier. Pictorial/video equipment as
described for approved diving companies shall be used to ensure that the surveyor is
satisfied with the information relayed to him.
ROV1 companies need not be approved as a service supplier, however the standard of
inspection should be equivalent to that achieved by a diver.
Diving/ROV surveys are to be witnessed and the results to be to the satisfaction of the
attending surveyor.
1 ROV means Remote Operated Vehicle, carrying camera/video equipment used for inspection purposes
as an alternative to divers.
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Condition Description
Site conditions
and visibility
The site must have sufficient water depth and clearance to all sides to allow divers to safely
survey the underwater hull.
Weather conditions at site including wave height and sea current affecting vessel motions
must be sufficiently low to allow safe inspection. If not so – the vessel needs to be taken to
a sheltered area where weather conditions are acceptable.
Water visibility must be sufficient to allow divers or ROV photo/video to get clear
pictures/recordings to the satisfaction of the surveyor.
Degree of marine growth should be limited to make a proper general visual survey possible.
Necessary cleaning is required if this is not the case.
MOU Draught Floaters shall be at minimum light draft during survey. If the vessel cannot deviate from
operational draft during survey, all parts of the hull below the operational water line shall be
covered by the bottom survey.
Condition of the
unit
The owner must give a written confirmation that the MOU has no record of damage from
dropped object, collision or grounding since the last bottom survey.
There should be no current recording of substantial corrosion, abnormal deterioration or
damage for the underwater parts or areas subject to UWILD or bottom surveys.
Units to comply with class requirements and documented through class survey work
according to the IIP.
Scope of the in-water survey to cover all relevant items of the underwater hull and
equipment
For units where the FUI > 1 special consideration will be given to an in-water survey
Reporting Upon completion of the survey, an inspection report including pictures/video recordings shall
be produced by the inspection company and presented to DNV GL for review. Reporting
should include measurements of cathodic protection system performance at renewal survey.
In case of severe
finding /
incomplete
survey
In case of findings or failure to complete in-water survey properly, drydocking may still be
required
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4 INSPECTION SCOPE
4.1 General
The UWILD survey shall provide an adequate assurance that the unit’s bottom is in a satisfactory
condition and will remain so until next scheduled bottom survey.
Each survey shall involve a general visual inspection of the unit’s bottom. The following sub-sections
summarize the expected areas covered by the alternative survey method as applicable for all MOUs
(Section 4.2) and as specific for the different structure types.
For completeness it is noted that other surveys and inspections may be carried out at the same time as
the UWILD. Examples are surveys for the Position Mooring equipment, the survey of the productionsystem, Cathodic Protection measurement etc. Additional inspections that may be included with the
UWILD scope could be:
In water inspection of risers In water inspection and dimensional control of mooring chains and wires
4.2 Applicable for all MOU
Table 4 lists the scope of inspection as applicable for all MOUs.
Table 4 General survey scope for bottom survey of MOUs
Areas covered Inspection approach
Cleaning In order to carry out a satisfactory inspection, the areas to be inspected must be sufficiently
clean to permit meaningful examination and photograph/video by diver/ROV.
Overall / spot cleaning may be required to the satisfaction of the attending surveyor.
Areas where marine growth, etc. may have to be removed are around and inside sea chests and
sea valves, on rudder nozzles with connections, guard plates for propeller shafts and thrusters.
Hull structure A representative selection of the underwater hull, as defined in the approved plan, shall be
covered by the survey.
The selection should include sufficient zones/spots to cover the hull from stern to bow, including
side and bottom plating.
Areas susceptible to damage from supply vessels, mooring equipment, dropped objects etc. are
to be inspected.
If an in-water approach is accepted for consecutive bottom surveys, the survey selection should
aim at covering different zones/spots for each survey.
The general condition of the unit, actual age, surface protection and history are importantfactors to consider when deciding the actual survey coverage.
The inspection shall cover reported suspect areas from past surveys.
The extent of inspection should be sufficient to identify damage to the hull structure, hull
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Areas covered Inspection approach
coating and general condition of critical welds.
General requirements to NDT of critical connections are covered by the IIP, whereas additional
NDT may be required for critical connections as part of the bottom survey if requested by the
Surveyor.
Seawater inlets
(sea chests and
strainers), Sea
valves
Sea suction openings are to be confirmed clear.
Sea inlets and valves are to be blanked off and dismantled for inspection during renewal
surveys (once in five year period)
Valves to be checked for tightness
Other openings
below water line
To be visually examined either from outside or inside.
Inspection to ensure that valves are functional and can be fully closed with no leakage.
Thrusters Survey the thruster for damage and visible oil leaks. Survey the connection to the hull.
Corrosion
protection
Anodes
Visual inspection of a representative amount of galvanic (sacrificial) anodes.
Potential measurements (cathodic protection readings), when required at renewal survey, shall
be performed with a bathycorrometer. Equipment should be tested and calibrated before use.
Impressed current system
Initial reading before cleaning to assess effect of installed system.
Then to be cleaned and re-inspected.
The surveyor should request readings from the permanent monitoring system and attach a
representative copy to the survey report.
4.3
Column-stabilized units
The bottom survey shall cover the submerged parts of the semi-submersible hull. More details for the
inspection scope specific for CSUs are provided in Table 5.
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Table 5 Specific additional scope for Column-stabilised units
Areas covered Inspection approach
Hull structure
Pontoon bottom plate areas
Pontoon side plate areas (both
sides)
Pontoon top plate
Columns below transit water line
Bracings1
For general scope, see Table 2
All connections to be covered by survey, i.e.
foundations fairleads, anchor rack to hull.
For cathodic protection system specific attention
should be paid to blind spots / poor coverage due
to brace arrangement / layout.
Propeller
Visual inspection of propeller blade, propeller boss,stern tube.
Check for damage, corrosion, erosion/cavitation,
oil leakage, cracks etc.
Rudder Attachments of rudder (if applicable) to be cleaned
and examined.
Visual inspection of welding, rudder blades,
attachments, paint, bearing
Units permanently on location may be exempted
from measuring the clearance of the rudder
bearing.
4.4 Ship-shaped units
Bottom survey shall cover the submerged parts of the ship-shaped hull. More details for the inspection
scope specific for SSUs are provided in Table 6.
Table 6 Specific scope for ship-shaped units
Areas covered Inspection approach
Appendages, bilge keel etc Ends of bilge keels to hull to be cleaned and examined
General inspection of bilge keels.
1 Areas above transit water line including bracings and bracing connections which are accessible for dry surveys andcovered by regular class inspections are not subject to inspection as part of bottom survey. These structural items arecovered by class scope and are included in the IIP.
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Areas covered Inspection approach
Propeller
Visual inspection of propeller blade, propeller boss, stern
tube.
Check for damage, corrosion, erosion/cavitation, oil leakage,
cracks etc.
Rudder Attachments of rudder (if applicable) to be cleaned and
examined.
Visual inspection of welding, rudder blades, attachments,
paint, bearing
Units permanently on location may be exempted from
measuring the clearance of the rudder bearing.
Moonpools and turrets The extent of the survey in the moonpool area should be
clearly identified showing any areas that are inaccessible for
survey
4.5 Self-elevating units/Jack-ups
4.5.1 Introduction
This section starts by introducing the two separate surveys as DNV GL has defined to cover the bottom
survey. Subsequently the scope of each of these is detailed further. Conclusively, the section defines
how to proceed for units for a period more than 5 years on location.
4.5.2 Separate surveys
Different from floating units, the hull (deckbox) of a jack-up structure is elevated above sea and fully
accessible for inspection during operation. At the same time the legs are available when the jack-up is in
transit mode with legs fully elevated. With due regard of this specific situation, DNV-GL has organized
the bottom survey in two parts (ref OSS-104, Pt. 7, Ch. 3, Sec. 2.3):
1) Bottom survey; covering external hull/deckbox
2) Spudcan-leg survey; covering spudcans and the underwater areas of legs, together with their
connections
The separation gives more control in planning and execution of the survey, at the same time provides a
clear overview of the unit’s (inspection) status.
It is underlined that these surveys taken together complement the statutory bottom survey as defined
by the different regulations as defined in 2.1.
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In a typical situation on location, the separate bottom survey can be completed while the spudcan/mat is
partly or fully obscured below mud line. In this situation, the separate bottom survey can be completed
(and credited) where the spudcan-leg survey remains pending.
With acceptance from the Flag, drydock in line with Section 3.3 and the referred OSS-104 paragraph,
the spudcan-leg surveys shall postponed till the next rig move1,2.
4.5.3 Hull/Deckbox
The deck box bottom and side surfaces are normally subject to general visual inspection when the jack-
up is in elevated mode. Alternatively the bottom survey may be carried out dry during transit if the jack-
up is placed on a barge or in water by divers when in floating mode, ref. requirements given in Table 7.
The following areas of the structure and the following scope shall be carried out during the survey:
Table 7 Normal Bottom survey of self-elevating units
Areas covered Inspection Approach
Visual inspection of hull, external
plate areas
Visual inspection of the deck box from boat or adjacent platform.
Alternative inspection by divers when in floating/transit mode.
Corrosion protection (CP) Visual check of the anodes, since the upper hull is usually out of the
water the CP system is usually inoperative.
Sea valves These can normally be opened from inside while the rig is in
operation.
Full closing of valves is to be checked.
Legs down to the splash zone Visual survey from the hull.
4.5.4 Leg and Spudcan
Leg and spudcan become available for inspection when the unit is in transit mode and is thereforenormally inspected separately from the hull when lifting the legs. Table 8 specifies the scope of this
survey.
1 The next due date taken from the date when the survey was due
2 For all clearness, it is noted that the interval of the spudcan-leg survey is equal to the traditional bottom survey, i.e.two times in any five (5) year period, with an interval not exceeding three (3) years between examinations.
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Table 8 Normal Survey Scope Leg and Spud Can Survey
Areas covered Inspection Approach
Legs General visual inspection of legs during jacking operation.
Connections between vertical chords and braces/diagonals are
checked together with racks and rack plate welding.
Around the pin holes in cylindrical legs
Leg-well structure
Jacking gear support structure
Spudcans ROV/diver inspection of the underside of the spudcan to look for
gross deformation.
Visual inspection around piping penetrations
Visual inspection of the spudcan from the outside, alternative check
of spudcan bottom from the inside, if access from outside is not
possible
Spudcan and mat internal structure in way of leg support
Legs – lower part
Leg / spudcan connection
The lower part of the leg, i.e. up to level 4 (in way of the
lower/upper guides in transit) and leg / spudcan connection is to be
visually surveyed.
Corrosion protection
Visual inspection of galvanic (sacrificial) anodes above sea levelwhen in transit condition
Visual inspection of a representative amount of galvanic (sacrificial)
anodes by diver or ROV when in other than transit condition
Alternative - potential measurements to evaluate efficiency of the
cathodic protection system when in elevated mode.
To be taken at different levels and sides of the leg to confirm
sufficient coverage.
4.5.5
Long-term operation at locationSelf-elevating units planning for long term operations on location beyond 5 years shall be designed as
permanently installed self-elevating units for the site specific environmental and soil conditions in
accordance to DNVGL-OSS-104, Pt. 7, Ch. 6. A separate approval process for UWILD applies for this
condition.
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APPENDIX A EXAMPLE OF PLAN DOCUMENT - TABLE OF CONTENT
The following figure represents a table of content of a plan document to be used for UWILD planning.
1. Introduction
1.1. Main Project/Vessel Particulars
Vessel Name:
Vessel Type:
Flag:
Class:
Vessel Owner / Operator:
Field:
Field Operator: Vessel Location
Vessel Summer Draft
Water Depth:
Project Launch Date:
2. Survey Procedure & Scope
2.1. General
Short description of scope, DNV GL’ s role/supervision, use of vendors
2.2. Planning
2.3. Hull Conditions
Damage/leaks/corrosion/coating
2.4. Weather/sea condition limitations
2.5. Cleaning methods hull/sea-chest
2.6. Use of Support vessel/Divers/ROV/CCTV etc.
2.7. Diving/ROV company
2.8. Items to be inspected
2.9. NDT methods to be used if required
2.10. Recording of survey and findings
2.11. Approval of vendors to be used
3. Project Team
Responsibilities
4. General
4.1. Client /OIM/etc.
Description of equipment/tools to be used
Procedures to be used to ensure safety during inspection
Risk assessment
Project Procedures & Reference Documents
- o0o -
Top Related